Where North Koreans have restricted access to exit North Korea, but South Koreans and probably foreigners having unrestricted access to the whole peninsula

So, the North plays Palestine to the South's Israel? Yeah, that'd really sell in Pyongyang!

This is assuming that the North collapses. Unification is gonna' occur forcefully, either through a collapse or an invasion. I think the most probable scenerio is North Korea collapsing through a coup organized by disgruntled generals. It also will probably occur at the worst time, maybe in the middle of a presidential election in the south, or during a big event the south is hosting (eg Olympics, World Cup).

I once heard that reunification or rather the cost to bring N. Korea up to S. Korean standards would cost 1.3 trillion US dollars. There is no way that S. Korea could afford to pay this alone. I'm sure a bunch of countries would be willing to throw in a few billion here and there but there is still a lot of money lacking. The States and China would be major donars but that still wouldn't be enough.
I think that reunitfication will happen one day but how it happens will be important. Assuming it happens in a peacefull way, will the N. Koreans (who will be wanting to have everything the S. koreans have) be patient with a slow reunification process that would limit the impact on the S. Korean economy or will the demand a qucik fix. I would guess that would want a quick fix and then things would turn pretty ugly.